Voltage regulators are used to automatically regulate an irregular voltage to a DC voltage of constant level. Generally, the voltage regulators can be classified into two operational modes: linear and switching modes. The linear-mode voltage regulator mainly includes an LDO (Low-dropout) regulator, while the switching-mode voltage regulator mainly includes a PWM (Pulse-width-modulation) regulator. A linear-mode voltage regulator usually provides a less output current, which is applicable to the usage of light load or no load, preferably to the light load. Instead, a switching-mode voltage regulator usually provides a larger output current, which is applicable to the usage of heavy load.
Voltage regulators operable in both linear and switching operational modes have been developed recently, in order to lower the fabrication cost in consideration of mess-production. An individual integrated-circuit (IC) chip of voltage regulator with both modes may be applicable to various user requirements. However, more electrical components would be included in the circuit layout of such a voltage regulator. The number of components needs to be decreased to lower the cost without degrading its operational performance.